For most applications amplifiers are designed to have an output impedance which is either very low (voltage source) or very high (current source). However, in certain applications an intermediate output impedance is desirable. One such application arises in telecommunications applications when amplifiers are required to drive a coaxial cable transmission line.
Probably the simplest way to match a transmission line is to drive the line with a voltage source in series with a resistor. The voltage source has a source impedance which is much less than the series resistor and the impedance of the series resistor is the same as the characteristic impedance of the transmission line. The disadvantage of this approach is that the signal out of the voltage source must be twice as large as the signal into the transmission line. In most telecommunication applications the voltage into the transmission line is specified, and the doubling of this voltage is very difficult with low supply voltage IC technologies.
Therefore it can be appreciated that an amplifier with a controlled output impedance which matches the impedance of a transmission line and which does not require twice the output voltage is highly desirable.